The invention relates to recording and/or reproducing apparatus, and in particular to apparatus of the type in which a disk-shaped flexible recording medium is mounted for rotation above a stationary stabilizing backing plate and, in operation, is supported by an air bearing produced by the flow of air between the rotating medium and a bearing surface of the backing plate.
When not in use the flexible disk lies at rest in contact with the backing plate. Accordingly, during start/stop operations there is a period before the air bearing is fully established during which time the disk is moving with its surface in contact with the backing plate. This can cause various problems depending upon the nature of the surface of the backing plate.
For example, if the backing plate is provided with a roughened sand or grit blasted surface as is often done to trap air between it and the disk in order to facilitate the establishment of an air bearing during start-up, then the contacting surface of the disk can and often does suffer from wear problems. The surface finish of a plate treated in this way is extremely difficult to control and even changes from plate to plate as the grit size changes during the blasting operation. Furthermore, pieces of grit can become impounded in the surface of the backing plate and although the average peak-to-peak surface finish is around 60-65 microinches, local peak-to-peak irregularities can be in excess of 100 microinches. Thus, in addition to the general wear of the disk surface these localized asperities produce quite deep scratches which, if this surface is used for recording and for reproducing operations, can cause errors.
If at the other extreme the backing plate is polished to a mirror-like finish, then the absence of trapped air causes the disk to strongly adhere to the surface of the backing plate. This phenomenon is known as "wringing" and the forces involved can be large enough to prevent a small drive motor even from starting to rotate the disk. If the motor is powerful enough to overcome the forces then distortion of the disk can still occur which adversely affect the subsequent operation of the apparatus.
Between these two extremes there are varying degrees of surface roughness which can be selected to trade-off the problem of "wringing" associated with a smooth plate against the problem of wear associated with a rough plate. This invention seeks to provide a backing plate with a surface finish which has the advantages of the two extremes but does not suffer from the disadvantages.